The metabolism of most drugs involves several phases whereby the xenobiotic substrate is usually rendered more water-soluble through a process of oxidation followed by conjugation. A great deal of work has allowed for our understanding of the first phase in the processing of xenobiotic through hepatic microsomal cytochrome P-450-dependent systems. Our laboratory has played a major role in the identification of certain second phase reactions, UDP-glucuronyltransferase activities. These enzymes catalyze the covalent linkage of drugs with glucuronic acid. The question of whether there are one or many UDP-glucuronlyltransferase activities can now be answered; there are many. Furthermore, we have discovered that there are isozymes present in rat liver endoplasmic reticulum which can catalyze the glucuronidation of a single substrate. The current application will address questions dealing with the separation and characterization of UDP-glucuronyltransferase activities in rat liver and focus on the number of isozymes available to catalyze the glucuronidation of drugs and steroids. New techniques have been developed which allow for the separation and characterization of isoforms dealing with estrogenic substrates, androgenic substrates, drugs and related substrates. Certain specific isoforms are induced following the chronic administration of 3-methylcholanthrene. Other inducing agents will be studied. Specific antibodies will be raised to the various forms of these enzymes. Antibodies may be used in both biochemical and immunohistochemical studies, and immunohistochemical localization of various UDP-glucuronyltransferase activities will demonstrate the locality of a given enzyme in a given tissue. Preliminary studies have already been successful. This investigation will advance our knowledge of the pharmacology and toxicology of the second phase of drug metabolism, that dealing with conjugation mechanisms available to the animal organism.